Andrew Nottingham, Alexander W. Cheesman, Terhi Ruitta, Christopher Doughty , elizabeth telford, Walter Huaraca Huasco, Noreen Majalap, Yadvinder Malhi, Patrick Meir, Yit Arn Teh

• Tropical forests are the most productive terrestrial ecosystem, fixing around 41 Pg of carbon from the atmosphere each year. A substantial portion of this carbon is allocated belowground to roots and root-associated microorganisms. However, there have been very few empirical studies on the dynamics of this transfer, especially in tropical forests where the response is mediated by high plant diversity.
• We used a large-scale girdling experiment to halt the belowground transfer of recent photosynthates in a lowland tropical forest in Borneo. By girdling 209 large trees in a 0.48 ha plot, we determined: i) the contribution of recent photosynthate to root-rhizosphere respiration and; ii) the relationships among the disruption of this belowground carbon supply, tree species composition and mortality.
• Soil CO2 emissions declined markedly (36 ± 5%) over ~50 days following girdling in three of six monitored subplots. In the other three subplots there was either a marginal decline or no response of soil CO2 emissions to girdling. The decrease in soil CO2 efflux was higher in subplots with greater dominance of Dipterocarpaceae.
• Mortality of the 209 trees was 62% after 370 days, with large variation among species. There was particularly high mortality for Dipterocarpaceae species. Whilst species with functional traits associated with faster growth rates (including lower wood density) had a higher risk of mortality post-girdle treatment.
• Overall, our results indicate a strong coupling of belowground carbon allocation and root-rhizosphere respiration in this tropical forest but with high spatial variation driven by differences in plant community composition, with a closer above-belowground coupling in forest dominated by Dipterocarpaceae. Our findings highlight the implications of the diverse species composition of tropical forests in affecting the dynamics of belowground carbon transfer and its release to the atmosphere.